I don’t know of any Indian Buddhist sect which denied the jātaka and avadāna-style literature with their presentation of the bodhisattva ideal. Do you know of one?
In terms of the dating around King Ashoka, this is based partly on things such as archeology (stupas, inscriptions, etc.) and written documents found, and otherwise based on scholars who analyze the history and development of ideas and tropes in literature. I’m offering generalized statements about the picture that knowledgable scholars give which I’m familiar with.
The Theravāda school accepts the jātakas, avadānas, bodhisattva path, etc. I put ‘Mahāyāna’ in brackets because it is a vague idea, but one often contrasted with schools like Theravāda or other Abhidharma schools like Sarvāstivāda. Are you saying it’s not a fact that there was a general trend amongst Buddhist schools, even ones not often considered Mahāyāna, to discuss these ideas? Are you suggesting we know absolutely nothing whatsoever about anything except the randomized contents of Buddhist documents available now? And that even still we know absolutely nothing about what the schools that preserved and commented on those documents thought?
I haven’t done extensive research, but what we see is something that evolves amongst schools that took the sutras as authoritative in their commentaries and abhidharma texts, and are found only in limited cases of sutras. By a scanning of the available evidence, it seems the idea evolved but relatively early on. It is a hypothesis.
‘Relative’ to what we can know, ‘well’ in terms of proportion of evidence leaning one way or the other, ‘established’ by people who research and discuss these points with some degree of rigor.
Scientists have pretty good ideas about evolution or the development of human culture and society, but some of it is also (AFAIK) still very vague in terms of particular dates or specifics. It’s possible for there to be reasonable trends and developments in a body of literature and religious movement and yet not have clear dates. It doesn’t need to be absolutely certain or sheer skepticism.
I hope that is somewhat clarifying. I’m just giving conversational, general statements about trends among scholars of Buddhism. This isn’t an academic article, and so I didn’t substantiate it to the level of one.